MARG sensor, which stands for the combination of magnetometer, accelerometer, and gyroscope, is widely used for three-dimensional attitude measurement. Among the mainstream solutions for MARG-based attitude estimation, complementary filter (CF) is normally regarded as a simplified alternative to Kalman filter (KF), mainly because CF can save the amount of calculation. A dual-vector discrete-time complementary filter (DV-DTCF) and its tuning methods are introduced in this paper. Different from the quaternion-based attitude estimation algorithms, DV-DTCF has linear measurement model, since it utilizes the gravity and geomagnetic vectors as its state variables instead of quaternion. This feature of DV-DTCF can avoid linearization error or the use of nonlinear algorithms, and can also greatly reduce its computational complexity. More interestingly, it is analytically revealed, and is also experimentally proven, the proposed DV-DTCF is fully equivalent to a fixed-gain KF. This fascinating fact straightforwardly leads to the tuning methods of DV-DTCF via the corresponding fixed-gain KF and Riccati equation. Such tuning methods of DV-DTCF are based on the statistic characteristics of MARG sensor noise, and that makes them solid and feasible. According to experiment results, DV-DTCF can achieve the same accuracy as that of commonly used KF algorithms in MARG-based attitude estimation, but with much lower time consumption. Hence, the proposed DV-DTCF is especially suitable for the applications that have strict limitations on computational costs.